1. Field of the Invention
The present invention relates to cooling computers and other electronic systems.
2. Background of the Related Art
One aspect of thermally managing computer systems involves properly cooling computer equipment to ensure the reliability and proper performance of that equipment. Properly cooling computer equipment typically requires generating airflow through computer equipment to remove heat generated by the computer equipment and maintain the temperature of various components within a suitable operating range. Innovations in computer technology have led to an increase in the power consumption and power density of computer equipment, along with increase in the airflow rates that are required to cool the computer equipment. As a result, the cost of operating computer equipment is also increasing. For example, it is now common for the costs associated with powering a server during the server's lifetime to exceed the original purchase price of the server. Total cost of ownership of computer equipment is an important consideration in the design of computer systems, and minimizing the total cost of ownership is therefore desirable.
Various fan configurations are available for cooling computer systems. Current systems and methods include the use of redundant, serially-arranged cooling fans. For example, US Patent Application 20070081888 to Harrison discloses the simultaneous use of two serially-arranged fans with variable-pitch blades that collectively induce airflow through the computer at rates sufficient to cool the computer. In the event of a failure of the primary fan, the primary fan blades move from an oblique position to a coaxial position, which aligns the blades with the airflow to present lower impedance as seen by the secondary fan. Harrison further discloses a control system to sense failure of the primary fan and increase the speed of the remaining secondary fan accordingly, in order to ensure that a minimum airflow requirement is met until the defective primary fan can be replaced.
Each individual fan in a typical set of serially-arranged fans will contribute to the overall airflow. Typically, each individual fan is unable to supply the cooling needs of the system as a whole, particularly at higher loads. Thus, multiple fans are operated simultaneously to achieve the airflow necessary to cool the system.
The problem with operating multiple fans is that each fan presents airflow impedance to the other serially-arranged fans. This impedance represents a loss component that substantially undercuts the efficiency of the cooling system. Even though Harrison teaches the use of variable-pitch fan blades having the capacity to be moved to a lower-impedance position, the pitch of a fan is only moved to the lower-impedance position in the event of a failure of that fan. During normal operation, however, each of the multiple fans is operated in the higher-impedance position in order to provide sufficient airflow to cool the system. Thus, no appreciable efficiency benefit directly results from the presence of variable-pitch fans in the configuration taught by Harrison.